The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Nondestructive testing (NDT) generally involves introducing an external excitation into an object to examine the object without permanently altering the material properties of the object. Thermography is one nondestructive testing technique and uses an external source of energy to heat the object and induce a temperature difference between defective and non-defective areas in the object. One common thermographic testing technique uses optical excitation where a pulsed light from a flash lamp such as a xenon flash lamp is transmitted to the object. Upon reaching an exterior surface of the object, the light is transformed into heat, which, in turn, propagates into interior of the object in the form of a thermal wave. The propagation of the thermal wave will be impeded and reflected back to the exterior surface of the object when the thermal wave hits a defect or a void in the object. The reflected thermal wave causes a local heat rise on the exterior surface. An infrared camera may be used to capture the thermal image, and namely the thermal pattern on the exterior surface. By analyzing the temperature differences on the exterior surface, the location of the defects or voids can be determined.
The typical thermography by optical excitation has its limitations. The ability of the flash lamp to introduce a thermal wave into the interior of the object is highly dependent on the optical properties of the object to be examined. An object with high reflectivity and low transmissivity can easily reflect the pulsed light away from the exterior surface of the object and allow less heat to be transformed and enter the object. As a result, the heat entering the object may not be sufficient to cause a surface temperature difference in an acceptable range to be easily visible with a thermal imaging camera.
Moreover, thermography by optical excitation is easily affected by environmental reflections and surface geometry and cannot be used for an object that has a complex shape and curvature.